1. Field of the Invention
This invention relates to downhole apparatus. In particular, but not exclusively, the invention relates to percussive drilling apparatus and a percussive drilling method.
In drilling operations in the oil and gas exploration and extraction industries drilling fluid or xe2x80x9cmudxe2x80x9d is pumped from the surface through the drillstring to exit from nozzles provided on the drill bit. The flow of fluid from the nozzles assists in dislodging and clearing material from the cutting face and serves to carry the dislodged material through the drill bore to the surface. It has been recognised that providing a pulsing fluid flow from the nozzles may also serve to increase the drilling rate. Further, it is well known that providing a percussive or hammer effect tends to increase the drilling rate.
2. Description of Related Art
Apparatus utilising one or both of these principles is described in U.S. Pat. No. 2,743,083 to Zublin, U.S. Pat. No. 2,780,438 to Bielstein, and U.S. Pat. Nos. 4,819,745, 4,830,122, 4,979,577, 5,009,272 and 5,190,114 all to Walter. A pulsing fluid flow is achieved by periodically restricting the drilling fluid flow area through the apparatus, the restriction creating a pressure force which provides a percussive effect. Flow restriction may be achieved by a variety of means, including valves which rotate about the longitudinal axis of the string, valves which rotate about a transverse axis, axially reciprocating valves and flap valves. The valve members are driven or reciprocated using drilling fluid driven motors comprising turbines of various forms, or fluid pressure fluid forces created by the movement of the valve member in the flow of drilling fluid.
In the majority of the earlier proposals, the pressure force which provides the percussive effect acts through a shock sub mounted above or below the flow restricting valve and the valve motor. The cycling fluid pressure causes the shock sub to extend and retract. However, such arrangements tend to be bulky. The movable part of the assembly has a significant mass, such that there is considerable inertia to be overcome with each fluid pressure cycle.
It is among the objectives of embodiments of the present invention to provide percussive drilling apparatus which is of relatively simple construction and is lighter and more compact than existing arrangements, to provide for more effective and efficient drilling.
According to a first aspect of the present invention there is provided a downhole drilling assembly comprising:
a body for mounting on a downhole string and defining a fluid conduit;
a drive motor mounted in the body;
a flow restricting device axially movably mounted to the body and for connection to a drill bit; and
a coupling between the motor and the flow restricting device,
the device being driveable by the motor to vary the flow of fluid through the body and produce a varying fluid pressure force to induce axial movement of the device relative to the body.
According to another aspect of the present invention there is provided a percussive drilling method comprising the steps:
mounting a drive motor in the body of a device mounted on a drill string;
mounting an axially moveable drilling fluid flow restricting device in the body and coupling the device to the drive motor;
coupling a drill bit to the device;
passing drilling fluid through the drill string and device; and
driving the device with the motor to vary the flow of fluid through the device and produce a varying fluid pressure force above the device, the force inducing movement of the device and drill bit relative to the body.
Preferably, the flow restricting device is connected to a mandrel or shaft having means for mounting a drill bit thereon. A mandrel may be splined or otherwise coupled to the body. Alternatively, a shaft may be rotatable relative to the body, such that the motor may also rotate the drill bit, providing a drilling motor capable of percussive or hammer drilling. In a preferred embodiment, the mandrel or shaft is spring mounted in the body.
Preferably also, the flow restricting device comprises a portion which is rotatable relative to the body and a portion which is fixed against rotation. In embodiments where the motor is utilised to drive a drill bit both the drill bit and the motor may be connected to the rotatable portion, and in embodiments where the drill bit is not driven by the motor the bit may be connected to the fixed portion.
The flow restricting device may be in the form of hollow shaft defining flow ports, the shaft being rotatable relative to a lobed sleeve; in one embodiment fluid flows into the hollow shaft via the flow ports, and when the flow ports are located behind the lobes of the sleeve the flow of fluid into the shaft is restricted. Alternatively, the device may include first and second valve members each defining a respective axial flow opening and which openings are aligned to collectively define an open axial drilling flow port through the device, the first member being rotatable about a longitudinal axis of the body to vary the alignment of the openings and thus vary the open area of the port.
Preferably also, the drive motor is drilling fluid actuated, for example the motor may be a positive displacement motor or a turbine.
According to an additional aspect of the present invention there is provided a downhole drilling motor assembly comprising:
a body for mounting on a downhole string and defining a fluid conduit;
a drive motor mounted in the body;
a flow restricting device mounted in the body;
a drill bit; and
transmission means for coupling the motor and the flow restricting device and for coupling the motor and the drill bit,
the device being driveable by the motor to vary the flow of fluid through the body and produce a varying fluid pressure force on the drill bit.
According to a further additional aspect of the present invention there is provided a percussive drilling method comprising the steps:
mounting a drive motor in a drill string;
mounting a fluid flow restricting device in the string;
mounting a drill bit on the string;
coupling the device and the drill bit to the drive motor;
passing drilling fluid through the drill string and the device; and
driving the device with the motor to vary the flow of fluid through the valve and produce a varying fluid pressure force to induce movement of the drill bit relative to the string.
According to a further aspect of the present invention there is provided a downhole tool comprising: a fluid transmitting body; a mandrel telescopically spring mounted in the body; and a flow restricting device mounted to the mandrel.
According to a still further aspect of the present invention there is provided a method of providing a cycling force in a drill string, the method comprising the steps of:
providing a downhole tool in the string, the tool comprising a fluid transmitting body, a mandrel telescopically spring mounted in the body and a flow restricting device mounted to the mandrel;
pumping fluid through the body; and
varying the configuration of the flow restricting device in a cyclical manner to vary the flow through the device, whereby a cycling fluid pressure force acts on the device and mandrel.
Preferably, in use, an increase in the fluid pressure force acting on the flow restricting device tends to extend the mandrel from the body.